The kidney histopathological spectrum of patients with kidney injury following snakebite envenomation in India: scoping review of five decades

Introduction Snakebite is a public health problem leading to about 58,000 deaths every year in India. Kidney injury subsequent to snakebite envenomation is common with a reported prevalence of up to 32%. The current study aims to elucidate the spectrum of kidney histopathology in acute kidney injury (AKI) cases associated with snake bites. Methods We searched seven electronic database studies to identify studies describing the histopathological findings in the kidney with snakebite envenomation. Two reviewers independently conducted titles and abstract screening as well as full-text evaluation for the final inclusion decision. Data were extracted as per the standardized form. We conducted narrative synthesis. Studies done exclusively on autopsy findings, in vitro studies, and case reports were excluded. Results We retrieved 1464 studies and finally included 28 studies which met the eligibility criteria in the analysis. Most studies were single-centre and the majority were cross-sectional. Overall we included a total of 534 renal biopsies. Russell’s viper bite was the most common cause related to AKI. Acute tubular necrosis was the most common finding followed by acute interstitial nephritis, acute cortical necrosis (ACN), and thrombotic microangiopathy (TMA). Vasculitis changes in vessels were rarely reported. Lesions such as ACN and TMA were associated with poor outcomes. Conclusion This analysis supports the notion that renal biopsies are important to guide prognosis and increase our knowledge about post-snake bite AKI pathophysiology. Supplementary Information The online version contains supplementary material available at 10.1186/s12882-024-03508-y.


Introduction
India has one of the highest burden of snakebite globally with an estimated 58,000 deaths per year from 2000 to 2019 [1].Half of the global deaths due to snakebite envenomation are from India [2,3].Acute kidney injury (AKI) after snakebite envenomation is an important cause of mortality and morbidity.The incidence of AKI after snakebite envenomation as a cause of communityacquired AKI has been reported to be as high as 26% in some Indian studies [4].Overall snakebite is a significant public health issue in many states of India where its incidence and consequent mortality and morbidity, remain high, although poorly understood [5].
Kidneys are highly vascular organs and therefore are more susceptible to snake venom-induced injury.AKI after snakebite envenomation may be associated with snake venom-induced injury mostly due to hemotoxic or myotoxic snakes of the Viperidae, Atractaspidae, Elapidae, and Colubridae families [6].In India Daboia russelii and Echis carinatus are common snake species known to cause AKI [7].
Patients with snakebite envenomation may clinically present with oliguria, anuria, proteinuria, haematuria, and advanced kidney dysfunction which may require dialysis [6].Enzymatic toxins found in snake venom contribute to damage across various kidney cell types, affecting glomerular, tubulo-interstitial, and kidney vasculature.While acute tubular necrosis (ATN) is the primary renal pathology, snake envenoming is also significant contributor to acute renal cortical necrosis, often exhibiting varying severity [6].The aftermath of AKI resulting from a snakebite can lead to diverse outcomes, ranging from death to nonrecovery, partial recovery and complete recovery of renal functions.Despite numerous studies focusing on non-invasive urinary and serum biomarkers for early AKI recognition in high-risk situations, differentiation between prerenal AKI and acute tubular necrosis, and prediction of AKI outcome, there is a notable scarcity of research on the utility of biomarkers in assessing long-term outcomes following AKI.While AKI after snakebite envenomation has been described in the literature, no systematic analysis of the literature on histopathological spectrum and the long-term outcome related to kidney injury in India has been done previously.We aimed to fill this gap by conducting an evidence synthesis of studies documenting renal histopathology findings after snakebite envenomation focusing on its relationship with pathophysiology, clinical findings, and prognosis.

Methods
We included studies that met the following criteria: • Type of participants: studies which included humans bitten by a snake (any) in India.• Concept: studies that report on histopathology (biopsy findings) of the kidney will be included.
Studies done exclusively on autopsy findings were excluded.• Study design -studies were included irrespective of study design, with the exception of in vitro studies, and case reports.• Restrictions: no restrictions on language or date of publication.

Data search
We searched electronic databases to identify studies describing the histopathological findings in the kidney with snakebite envenomation from India.We searched seven electronic databases (MEDLINE,EMBASE, CEN-TRAL, Global Health, PsychINFo, EMCare, SafetyLit) until May 2023 to search for studies and supplemented it with manual screening of references list of included studies.The search strategy was as follows: [(Snakebite) OR (Snakebite envenoming) OR Snake bite induced acute kidney injury)] AND [(renal histopathology) OR (kidney histopathology)) OR ((kidney biopsy) OR (renal biopsy)].

Article selection
Articles were chosen via a two-step process.At least two independent authors screened each record based on titles and/or abstracts and marked each record as "exclude" or "include " in a cloud-based artificial intelligence-guided platform (Rayyan -https:// www.rayyan.ai/).Disagreements at this phase were resolved by consensus.If there was a consensus that an article was unsuitable for inclusion based on the title and/or abstract, it was excluded.Subsequently, two authors conducted an independent screening of the full-text articles and only those that received agreement from both authors were included.
In cases where consensus was not initially established, a third author was consulted for discussion until a consensus was achieved.The data was extracted as per a predesigned data extraction form by two authors and then verified by two other independent authors.We synthesized the data narratively using data as reported in the primary studies, without conducting any additional statistical analysis.

Data extraction
Data collection included the study population size, species of snake responsible for AKI, the state in which the study was conducted, the timing of the renal biopsies, the prognosis on long-term follow-up, proportion of survivors, and detailed description of histopathological findings [including light microscopy (LM), Immunofluorescence (IF) and Electron microscopy (EM)] provided in the studies.The renal prognosis of the patients was categorised into 3 categories: (1) persistent renal dysfunction at discharge, (2) progression to end-kidney renal disease and (3) hemodialysis dependency.For the studies providing the long-term follow-up data, follow-up duration was also collected and presented.Only patients with renal biopsy showing signs of TMA such as fibrin thrombi in glomeruli and arterioles were included in the manuscript.

Selection of sources of evidence
We retrieved 1464 articles (after removing duplicates) but finally included 28 articles which met the inclusion criteria.The PRISMA flowchart showing the inclusion of studies is presented in Fig. 1.

Characteristics of included studies
The characteristics of the included studies are shown in Table 1 .All studies were single-centre with most of them being cross-sectional in nature.Overall we included a total of 534 renal biopsies and 107 renal autopsy findings.Supplementary appendix 1 shows PRISMA-ScR Checklist for the reported studies Two studies, [22,31] were exclusively done on children with a mean age (± SD) in years of 11.52 ± 2.88 years and 5.8 ± 1.0 years respectively.In the majority of studies, AKI was caused by Russell's viper, however, in some studies, Echis carinatus and sea snakes were also identified to be associated with kidney involvement [15-17, 27, 29].

Timing and indication of renal biopsy
In the majority of studies, kidney biopsies were done after 2-3 weeks from the onset of AKI, The most common indication for renal biopsies was persistent renal dysfunction or dialysis dependency.Pathological changes varied depending upon the time lag from the onset of AKI to kidney biopsy.Histopathological findings in Kidney biopsies of included studies are shown in Table 2. Kidney tissue obtained during the early diuretic phase of acute tubular necrosis (ATN) revealed epithelial degeneration in tubules, tubular vacuolation, desquamation, and severe intertubular interstitial oedema with regenerative changes developing at the later stages of AKI (after 3 to 8 weeks) [9,30,31].Interstitial haemorrhage was more common in the 1st week of the bite [15].The nature of the tubular casts also changed with the time lag between the bite and kidney biopsy.Hyaline casts with degenerating cellular-granular casts were more commonly

Glomerular lesions
Changes in the glomerulus were largely underreported.Glomerular changes occurring with acute cortical necrosis (ACN) were the most commonly reported finding.However, thrombotic microangiopathy (TMA) and focal and diffuse mesangial proliferation have also been reported [30,32].Studies also reported necrosis of glomerular tuft and isolated glomerular thrombosis [9].Specific glomerular changes like ballooning and dilatation in the glomerular capillary loops, focal proliferation of mesangial cells, endothelial cell swelling, splitting of glomerular capillary basement membrane were reported by Mittal et al. and Acharya et al. [15,17].

Tubulointerstitial changes
ATN was the most frequently encountered finding in kidney biopsy tissue.The incidence varied from 30 to 100% [8, 9, 11, 12, 14-16, 18, 23, 25-27, 32, 34].Hyaline, granular, or pigment casts were frequently seen along with dilated tubules lined with flattened epithelium and desquamation of necrotic cells [12,28].Acute interstitial nephritis (AIN) has been reported non uniformly [25-27, 32, 34].In a series by Priyamvada et al., 5.7% of snakebite-induced AKI were reported to have AIN on kidney biopsy [26].The kidney biopsy demonstrated mixed infiltrate of predominantly lymphocytes and variable proportions of other cells like neutrophils, eosinophils, and occasional plasma cells.Neutrophil cast was reported in one patient.The prevelance of AIN was slightly higher in other series; out of 85 biopsies, 20 (23.5%) patients had AIN [27].Marked infiltration of eosinophils and lymphocytes with substantial tubular injury was reported.Golay et al. observed extensive interstitial inflammation, with a predominant lymphocytic infiltration [20]).In viperine envenoming, "hemorrhagic interstitial nephritis" characterized by haemorrhages in the interstitium with tubular necrosis and RBC congestion in the tubular lumen was reported [10,17].One study reported features of pyelonephritis complicating ATN [17].Patchy and diffuse areas of hemorrhagic necrosis in the cortex and widespread medullary areas have also been reported in in Russell viper's envenomation [9].
Pigment induced nephropathy Sakthirajan et al., in their analysis of pigment-induced nephropathy, found snake envenomation as the most frequent etiology of rhabdomyolysis [28].Out of the 26 patients with rhabdomyolysis, 10 were caused by snakebite envenomation.All biopsies revealed features of acute tubular injury and pigment casts.In an another series, among a cohort of 56 patients diagnosed with hemoglobin cast nephropathy, the second most prevalent etiology, following drug-induced cases, was attributed to snake envenomation-induced hemolysis.This particular cause was observed in 16 patients, accounting for approximately 28.4% of all patients.Positive myoglobin immunostaining was observed in two patients who had suffered snake bites envenomation [35].

Vascular changes
Vasculitis-like changes including necrotizing arteritis, thrombophlebitis, and vessel wall necrosis have been described in Russell's viper bite cases [9,13,15].Severe congestion of large intrarenal vessels along with venules and veins and crowding of neutrophils in intertubular capillaries were reported by Chugh et al. [16].

Thrombotic microangiopathy
Rao et al. described a series of TMA in AKI induced by snake-bite envenomation [30].In this study, out of 103 patients post snake-bite envenomation AKI, 19 (18.5%) had clinical features of TMA.However, renal biopsy was done in only 2 patients which showed features of TMA such as fibrin thrombi in glomerular capillary lumen and arterioles with patchy cortical necrosis.Priyamvada et al. also reported chronic TMA in patients who developed CKD following snakebite envenomation [29].

Acute cortical necrosis (ACN)
Following ATN, ACN was the second most common finding reported in patients with snake-bite AKI.Its incidence varies between 5 and 100% of biopsies reporting it [10-13, 15-18, 21, 22, 24, 31, 32, 34].Fibrin and platelet thrombi were found predominantly in lobar and sublobar arteries.In an analysis by Chugh et al. including 113 cases of ACN, viperine snake-bite envenomation was one of the major causes responsible for 16 (14.2% ) of total ACN cases [18].

Immunofluorescence (IF)
Only a few studies reported IF findings [13,17,20,26,27].A study showed dense C3 deposits in the afferent and efferent arteriolar walls in cases of necrotizing arteritis [16].Mittal et al. demonstrated IF results in 7 patients and reported a weak positivity for IgG and IgM, along with C3 positivity in the mesangial area [17].Priyamvada et al. found a weak C1q deposition in the mesangium [26].Other studies failed to demonstrate any deposits in IF [20,27].

Electron microscopy
Studies describing electron microscopic findings were scanty [12,14].Date et al. provided a detailed description of ultrastructural findings.The authors reported swollen cytoplasm of bowman's capsule epithelium with visceral epithelium showing blebs, microvilli, patchy foot process fusion, and intracytoplasmic lipid vacuoles.The basement membrane of the glomerular capillaries was thick and wrinkled.In blood vessels, endothelial cells were swollen and cytoplasmic protrusions were seen to be protruding into the lumen.Infiltration of inflammatory cells was found in the interstitium.Intracytoplasmic bodies were seen in the proximal tubules representing degenerating organelles.

Renal biopsy findings and outcomes
The kidney outcomes in AKI following snakebite envenomation varied from partial or complete recovery of kidney functions to progression to end-stage kidney disease (ESKD) resulting in dialysis dependence.Studies reported that snakebite envenoming patients who did not recover their kidney functions had diffuse cortical necrosis and TMA as the predominant pathological findings.Patients with acute tubular injury were reported to respond well to conservative management and dialysis contrary to those with ACN who only responded partially or not at all [10].Supplementary Table 1 shows renal and patient outcomes in biopsied patients.Sarangi et al. demonstrated that the clinical presentation and prognosis of the patients were directly proportional to the severity of renal histopathological lesion on the kidney biopsy [11].Lower survival rates were reported in ACN. 8 out of 10 patients (80%) who had bilateral renal cortical necrosis, and 4 out of 23 patients with less severe acute tubular lesions died (P < .001).Other series reported a mortality rate of up to 100% in patients with ACN [8].
Studies also reported that as compared to non-TMA cases of AKI, TMA cases were associated with more advanced azotemia at presentation with an almost universal requirement for dialysis.These patients required a longer duration of renal replacement therapy (RRT), and hospitalization and had higher chances of progressing into chronic kidney disease (CKD) with higher mortality insinuating a poor prognosis [29,30].Golay et al. reported worse clinical outcomes while comparing cases with and without AIN [23].

Discussion
To the best of our knowledge, this is the first systematic synthesis of evidence presenting an analysis of renal histopathology findings after snakebite envenomation in India.ATN followed by ACN were the common renal histopathological lesions reported in multiple studies but TMA, mesangial proliferation, pigment-induced nephropathy and AIN were also reported.Globally other histopathological findings such as proliferative glomerulonephritis after Echinatus carinatus bite and crescentic and diffuse proliferative glomerulonephritis after Russell's viper have also been reported [36][37][38].None of the studies in our analysis reported such changes.Studies included in our review show that snakebite AKI patients present with oliguria, hematuria, and advanced azotemia requiring dialysis, with persistent oliguria lasting for more than 2-3 weeks implicating the occurrence of ACN [10,13,18]. .The mechanism of kidney injury in snakebite envenomation is usually multifactorial, it is an interplay of various cytokines, vasoactive substances like endothelin, and other immune mediators [39][40][41].It can be attributed to numerous reasons such as direct nephrotoxicity of the venom, circulatory collapse, intravascular hemolysis with hemoglobinuria, extensive myonecrosis causing myoglobinuria, or Venom-Induced Coagulopathy (VICC).Various proteases, amino acid esterase enzymes, and hemorrhagic proteins present in viperine snake venom can activate procoagulant factors and induce coagulation cascade abnormalities including bleeding diathesis and VICC [39].Postulated mechanism in snake bite associated AKI is shown in Fig. 2 mechanism of snake.
The pathogenesis of VICC distinguishes itself from Disseminated Intravascular Coagulation (DIC).While DIC arises from multifaceted mechanisms culminating in fibrin deposition, VICC's coagulation activation originates primarily from a snake procoagulant toxin, not the tissue factor/factor VIIa pathway implicated in DIC [42].VICC's severity varies based on the toxin's action within the coagulation pathway, ranging from mild fibrinogen consumption to severe deficiencies in fibrinogen, factor V, and factor VIII. Crucially, VICC lacks evident fibrin deposition, microvascular thrombotic obstruction, and resulting organ damage, in contrast to DIC [43].VICC predominantly manifests as bleeding, with the risk determined by whether the snake toxin acts as a hemorrhaging agent, such as by metalloproteinase prothrombin activators.These activators not only trigger the coagulopathy pathway but also induce vascular injury, heightening the risk of bleeding, a distinctive feature absent in DIC where vessel walls remain unaffected [40].Pigmentinduced nephropathy was seen in the cases complicated with intravascular hemolysis leading to hemoglobinuria.It was particularly more common in viper and crotalid snakebites whereas sea snakes were categorically myotoxic [39].
The infiltration of various cells such as lymphocytes, monocytes, eosinophils, basophils, and mast cells in the interstitium in cases of ATN was hypothesized to be either a consequence of some immunologically mediated reaction to antigens released from necrotic renal tubules or due to homocytotropic antibody-mediated reaction [44,45].However, their activation and accumulation in the interstitium have also been argued to be a similar phenomenon as seen in delayed hypersensitivity reactions [46].It was postulated that lesions occurring in the early phase could be the consequence of the direct toxic effect induced by snake venom and the lesions developing late could be plausibly immunologically mediated resulting in the formation of fibrin thrombi similar to those in hemolytic uremic syndrome (HUS) and VICC [14].
Vasculotoxic effects and complement activation by the snake venom probably via alternate pathways (based on the C3 without immunoglobulin deposition on the arterial wall) are plausible mechanisms for the pathogenesis of these vascular lesions [17,26].
Kidney biopsies during the initial week are usually not feasible and pose a high risk due to coagulation abnormalities and thrombocytopenia.This is a deterrent in establishing a correlation between structural and functional findings in the kidneys.Kidney biopsy remains an essential tool to predict prognosis.ACN and TMA lesions impart worse prognosis and usually progress to ESKD and dialysis-dependent stages in most cases.The majority of studies have not adequately differentiated between outcomes associated with TMA versus non-TMA cases.Notably, the challenge of performing biopsies in TMA cases is compounded by factors such as advanced azotemia, low platelet count, and anaemia.Furthermore, TMA-like changes in blood vessels have been identified concomitantly with other lesions like ACN and ATN contributing to an overall portrayal of poor prognosis in these lesions.This inherent difficulty in obtaining biopsies and the overlapping nature of TMA with other renal conditions, collectively lead to a generalization of unfavourable outcomes in the existing literature, Additionally, studies like those conducted by Priyamvada et al. raise concerns about potential high selection bias, particularly in patients proceeding to renal biopsy.Mostly the patients with persistent renal dysfunction and having poor renal outcomes undergo renal biopsy.This bias may compromise the ability to conduct a comprehensive comparative outcome analysis between different types of renal histopathological lesions.The role of steroids and plasmapheresis in the management of AIN and snake envenomation-induced TMA respectively is yet to be elucidated.
Our review also indicated some gaps in evidence, which need to be investigated in the future.Most studies on the domain are cross-sectional in nature providing no insights on prognosis and other issues.As such information on the interpretation of pathophysiology, and exploration of treatments for kidney management is not possible.Interpretations around the utility of doing renal biopsies in the early phases of AKI in informing clinical practice are also not possible from cross-sectional Fig. 2 Proposed mechanism of snake bite-induced kidney injury VICC: Venom-induced consumption coagulopathy studies.Another key challenge is the lack of uniformity in reporting the cause of AKI by biopsy.Different histopathologic changes may have been attributed to the same disease process, redundancy in the nomenclature cannot be ruled out.There is a need for consensus standardisation in the domain.The absence of species identification and its consequential effect on the interpretation of the review's findings is another limitation.
The venom compositions of snakes differ both within and across species.This will lead to varying causes of AKI and, thus, potentially varied histological findings.

Conclusion
In conclusion, this scoping review represents the inaugural systematic synthesis of examining renal histopathology findings in snakebite envenomation-induced kidney injury in India over five decades.ATN and ACN were predominant histopathological lesions, with additional reports of TMA, mesangial proliferation, and AIN.Prognostically, ACN lesions signal poorer outcomes, often progressing to ESKD.Gaps in evidence, including limited longitudinal studies and the lack of standardized reporting, necessitate future research to enhance our understanding and inform clinical practice in managing snakebite-induced kidney injury.

Fig. 1
Fig. 1 The PRISMA flowchart showing the inclusion of studies

Table 1 (
continued) NR Not reported or not clear, HD Hemodialysis, ACN Acute cortical necrosis

Table 2
Kidney biopsy findings chart